LeetCode OJ-- LRU Cache ***@

https://oj.leetcode.com/problems/lru-cache/

涨姿势的一道题，相当于实现一种数据结构，实现最近最少使用数据结构。

// 用来记录 前后节点都是什么 类似链表上的节点
    struct DataNode{
        int key, value;
        DataNode *prev, *next;
        DataNode(int _key, int _value){
            key = _key;
            value = _value;
            prev = NULL;
            next = NULL;
        }
    };
class LRUCache{
public:
    int capacity;
    DataNode *head;
    DataNode *tail;
    //用来快速查找，key在不在。本来用stack最自然了，但是它不支持中间位置的删除元素操作
    map<int,DataNode*> cache;

    LRUCache(int capacity) {
        this->capacity = capacity;  
        head = NULL;
        tail = NULL;
    }
    void moveToHead(DataNode *p)
    {
        if(p == head) // p is head
            return;
        if(p == tail)
        {
            tail = p->prev;
            tail->next = NULL;

            p->prev = NULL;
            p->next = head;
            head->prev = p;
            head = p;
        }
        else
        {
            p->prev->next = p->next;
            p->next->prev = p->prev;

            p->prev = NULL;
            p->next = head;
            head->prev = p;
            head = p;
        }
    }
    //取得值，并把这个值的node挪到最前面，相当于使用了一次了
    int get(int key) {
        if(cache.count(key) == 0)
            return -1;
        DataNode *p = cache[key];
        moveToHead(p);
        return p->value;
    }
    //如果不存在，则插入到最前面。如果存在则设置值，并把这个值的node挪到最前面，相当于使用了一次了
    void set(int key, int value) {
        if(cache.count(key) == 0) //本来不包含
        {
            DataNode *p = new DataNode(key,value);
            cache.insert(make_pair(key,p));

            if(head == NULL)
            {
                head = p;
                tail = p;
            }
            else // p is new head
            {
                p->prev = NULL;
                p->next = head;
                head->prev = p;
                head = p;
            }
            // remove tail
            if(cache.size() > capacity)
            {
                cache.erase(tail->key);
                tail = tail->prev;
                tail->next = NULL;
            }
        }
        else
        {
            DataNode *p = cache[key];
            p->value = value;
            moveToHead(p);
        }
    }
};